in a day on a day区别

　　On a clear day you can see France.

　　天气晴朗时你可以看见法国。

　　There's nothing like a brisk walk on a cold day!

　　冷天出来快步走走，简直太舒服了。

　　There are only so many hours in a day.

　　一天不过这么几个小时。

　　In a day or so he will be as good as new.

　　他大概一天工夫就会恢复过来。